The available airspace used by both civilian and military aircraft today is getting more and more crowded by each year. As a result, there is an imminent need for onboard automatic systems able to detect possible collision threats and enable the manoeuvring of the aircraft away from such mid-air collision threats. Such onboard automatic systems are generally referred to as collision avoidance systems, and are especially critical for autonomous aircraft, such as, for example, unmanned aerial vehicles (UAVs) where there is no pilot onboard.
Unmanned aerial vehicles (UAVs) depend on its collision avoidance system to detect possible collision threats in order for the system to be able to calculate an optimal escape trajectory, and accordingly manoeuvre the autonomous aerial platform away from the possible collision threat. U.S. Pat. No. 6,510,388 describe a system for avoidance of collision between vehicles. The system may calculate and store a manoeuvre trajectory for a vehicle, which the vehicle is made to follow if a comparison shows that the avoidance manoeuvre trajectory of a vehicle in any moment during its calculated lapse is located at a distance from other vehicles that is smaller than a stipulated minimum distance.
A collision avoidance system may be based on passive sensors mounted on the aircraft, such as, e.g. a video camera. The advantages with using a video camera are numerous, such as, for example, the possibility of achieving high image frame rates, a very high resolution, and a sensitivity that is similar to or better than that of the human eye. By using a passive video camera, it is possible to achieve a tracking of targets which is generally referred to as “bearings-only-tracking”. This is because only the bearing of a target is retrievable from the outputs of the passive video camera. From the bearing a “time-to-go”-value, or TGG-value, may be calculated. The TTG-value may inform the collision avoidance system of the minimum time left until the detected target may be reached. The TTG-value may then be used to calculate an escape trajectory. Unfortunately, the result of such calculations is often inaccurate and approximate.
In O. Shakeria, W. Chen and V. M. Raska, “Passive ranging for UAV Sense and Avoid Applications”, AIAA-2005-7179-978, the feasibility of estimating a range and velocity of other air traffic by performing a small self-maneuver is investigated.